The field of the invention is that of the observation of geostationary satellites and is more particularly concerned with a system for observation of geostationary satellites, use of a system of this kind and corresponding observation methods.
By definition, a geostationary satellite is a satellite that has been placed in geostationary orbit, for example by a launch vehicle such as Ariane (France) or a space shuttle such as Discovery (USA). From any geographical location of the surface of the Earth, a geostationary satellite is always seen at the same azimuth and elevation coordinates. The geostationary orbit is the curved path followed in space by geostationary satellites. It is a quasi-circular orbit approximately 36 000 km from the Earth.
Geostationary satellites can have a civilian role (telephone communications, television transmission, meteorology, etc) or a military role (encrypted communications, detection and location of electromagnetic emissions, launching of missiles, etc) and this is why it is of benefit to be able to monitor the nature of their roles. For example, it is important to be able to detect whether a supposedly civilian satellite in fact has a military role. In particular, it is desirable to be able to distinguish between communication, meteorological, early warning and electromagnetic monitoring satellites.
Existing observation systems use imaging devices installed in ground stations. These imaging devices may use adaptive optic telescopes, for example. However, their main drawback is that the quality of the image they provide depends on the illumination conditions: they cannot be used in cloudy weather or in the presence of atmospheric turbulence. Moreover, the best optical and radar imaging systems operating from ground stations offer in fine weather decimetric resolutions at 300 km, i.e. decametric resolutions beyond 40 000 km; it follows that a satellite in geostationary orbit occupies only one pixel of an image. In this case, it is just possible to detect the presence of an intruder satellite in the geostationary orbit but it is impossible to determine the role of a geostationary satellite.
Furthermore, these telescopes are very costly and of not inconsiderable size.
Finally, for the meteorological reasons previously mentioned, these telescopes are located on exposed sites, for example on mountain tops, and therefore constitute vulnerable targets.